1. Field of the Invention
This invention relates to a method and apparatus for processing a thin film formed on a substrate using a laser beam.
2. Description of the Related Art
Laser devices are widely used in the processing of materials such as cutting, welding, surface treating, and removing of materials from the surface of a substrate. The removal of materials from the surface of a substrate requires applying a beam of laser light having a frequency within the absorption ranges of the material, either directly onto the material itself or through the surfaces of transparent substrate or a transparent film. The laser energy is absorbed by the subject material and converted to heat energy, whereby the material is removed by being thermally altered or vaporized. For example, Japanese Laid-Open Application Sho 61-14727 describes removing a thin film formed on the surface of a transparent substrate by applying a laser beam onto the thin film to vaporize or to initiate a peeling action to remove the thin film from the substrate surface. These technologies can be effective in the precise fabrication of large size substrates, and are used actively in pattern generation in the fabrication of plasma display (PDP) units and thin film solar cells.
Japanese Laid-Open Application Sho 57-1256 describes fabricating a solar cell using laser patterning technology to produce multiple series connections for rectangular stripe-shaped thin film solar cell elements to form an integrated structure. The integrated structure is formed by forming a plurality of rectangular stripe-shaped transparent electrodes on a transparent substrate, the electrodes being divided by a plurality of equally spaced first isolation lines. A semiconductor layer that converts light energy to produce an electromotive force is then formed on the transparent electrodes, and divided into stripe-shaped elements separated by a plurality of second isolation lines. The second isolation lines are equally spaced at the same spacing as the first isolation lines, and are located at positions adjacent but offset from the first isolation lines. A backside electrode layer is then formed on the semiconductor layer, and divided into stripe-shaped elements by a plurality of third isolation lines. The third isolation lines are again equally spaced at the same spacing as the first isolation lines, and located at positions offset from the second isolation lines on the opposite side of the first isolation lines. The neighboring elements of transparent electrode stripes and backside electrode stripes are connected at the areas of the second isolation lines. As a result, a structure having a plurality of elements connected in series is formed.
The integrated structure described above, as well as alternative structures where the backside electrode layer and the semiconductor layer are separated by a plurality of third isolation lines, have been used since the 1980s. Both of these structures are equipped with electrodes located at both ends of the row of the series-connected solar cell elements for the purpose of drawing out the generated electrical power. The edges of the substrate typically have unusable border areas of 5 to 10 mm in width that can not be used for power generating, since the physical framing of the module will shade out these border areas. In addition, a frame holding area is required during the semiconductor and backside electrode fabrication processes which also masks out a certain amount of the edge portions.
In the past, the transparent electrodes were fabricated from a film of ITO (indium-tin-oxide) using a conventional evaporation technique, but it is now more common to fabricate the transparent electrode with SnO.sub.2 (tin oxide) thin film using a process known as thermal CVD. Since the thermal CVD process can not be applied selectively using a mask, the entire substrate surface is therefore covered with the tin oxide layer. The electrode areas and power generating areas must therefore be electrically insulated from the peripheral boundary where electrical conductors are present. One way to achieve such insulation is to install isolation separator lines which partially remove the transparent electrode from the areas surrounding the power generating area and the backside electrode area. The width of the isolation separator lines should be a minimum of 100 .mu.m, so that when the solar cell elements are encapsulated with an organic molding compound, the insulation can withstand a voltage of 1500 V and function reliably. Japanese Laid Open Application Hei 8-83919 describes a method of producing isolation separator lines having proper widths by running laser scribing lines several times on the substrate surface.
Thin film solar cell modules fabricated as described above may be sealed with a vacuum lamination method by piling an ethylene vinyl acetate (EVA) type laminated polymer sheet and a polyvinyl fluoride polymer sheet (Tedlar.RTM.) on the thin film solar cells. The solar cell modules may be completed by attaching a terminal box and an aluminum frame.
As described above, laser processing is an extremely effective pattern generating tool in the fabrication of solar cells. However, a major drawback of this process is that it is slow and cumbersome, since a single laser beam is used with a single point source running on the substrate. In addition, if the material to be processed by the laser is thick, repeated applications of the laser beam will be required. This requires the moving mechanisms to be highly accurate and reproducible, increasing the complexity and cost of the processing devices.
Typically, a laser device is capable of producing continuous power generation. However, when the materials being processed require a highly amplified and focused beam, a giant pulse laser method is typically used. In a giant pulse laser process, the energy is stored in the laser medium, and the light is generated by a Q-switch when an adequate amount of energy is accumulated. Pulse lasers such as excimer lasers are ideal for the types of material processing described above, since pulse lasers can typically generate pulses only, and the laser generation only exists for short periods of time.
Laser processing may be carried out by moving the substrate relative to the laser beam. Recent advancements in mechanical devices have made it possible to move the substrate at a speed of 100 cm/sec. However, the pulse frequency of a Q-switch operated laser is limited. Thus, to produce a trace of overlapping light spots using a single high-intensity focused beam, the substrate must move at an appropriate speed determined by the optimum frequency of the laser pulses, which is typically slower than the speed attainable by the mechanical structure. This reduces the speed of the processing. For instance, to process a tin oxide layer applied on a glass surface using a commonly available neodymium YAG laser with a power rating of 2 W at 10 kHz and a beam spot of 50 .mu.m in diameter, the upper limit of the processing speed is about 50 cm/sec (50 .mu.m.times.10,000/sec). Because of the need to overlap the beam spots, the actual speed will be about 40 cm/sec. To increase the processing speed, the laser pulse frequency must be increased. For example, if the processing speed is to be doubled, the laser pulse frequency must be increased to about 4 W at 20 kHz. In addition, the amount of energy in each laser spot at the working site must be maintained. It is difficult to produce a laser device meeting such requirements. A YVO.sub.2 laser with a high Q-switch frequency of 100 kHz is available, but its power is an order of magnitude too low to meet the above power requirement.
While 50 cm/sec might appear to be a high speed, it is insufficient for solar cell processing. For example, to fabricate a 1 m.sup.2 solar cell module with 10 mm wide solar cell elements, a laser running distance of over 100 m is required, which theoretically takes 200 seconds to complete. In reality the cycle time is more like 5 minutes, making mass production problematic.
Moreover, the above-described processing speed can only be achieved for processing thin film layers of less than 1 .mu.m, where a single pass of the laser beam suffices. To process thicker layers, such as CdTe layers, or the increasingly popular polycrystalline silicon (poly-Si) layers which are typically over 2 .mu.m in thickness, the laser beam must traverse the same film area twice or more, further increasing the processing time. In addition, when the laser beam is applied two or more times over the same area, the beam must be accurately repositioned, typically to within 10 .mu.m from the previous laser spots. Such requirements increase the complexity and cost of the processing device.